Atopic dermatitis (AD) is a chronic inflammatory skin disease that affects 15 to 30% of children and ~5% of adults in industrialized countries. Although the pathogenesis of AD is not fully understood, the disease is thought to be mediated by an abnormal inflammatory response including immunoglobulin E (IgE) production in the setting of skin barrier dysfunction. Loss-of- function mutations in the Filaggrin gene encoding an epidermal protein that promotes the skin barrier, are strong predisposing factors for the development of AD. Because a hallmark of AD is an altered skin barrier, understanding of the mechanism by which Filaggrin deficiency increases the susceptibility to AD may provide critical insight into disease pathogenesis. Mast cells (MCs) contribute to IgE-mediated allergic disorders including AD. Upon activation with IgE and antigen or microbial stimuli, MCs release their membrane-bound cytosolic granules leading to the release of multiple molecules that are important in the pathogenesis of AD and host defense against bacterial pathogens. Notably, more than 90% of AD patients are colonized with Staphylococcus aureus in the lesional skin whereas the skin of most healthy individuals do not harbor the pathogen. Several Staphylococcal exotoxins (SEs) can act as superantigens and/or antigens in models of AD. However, the role of these SEs in disease pathogenesis remains unclear. We have identified ?-toxin, a peptide released by S. aureus that belongs to the peptide toxin family of phenol-soluble modulins (PSMs), as a potent inducer of MC degranulation. ?- toxin is a member of the family of phenol-soluble modulins that is produced by the virulence Agr quorum sensing of S. aureus. Importantly, S. aureus isolates recovered from AD patients produce high levels of ?-toxin. Notably, skin colonization with S. aureus, but not a mutant deficient in ?-toxin, promoted IgE and IL-4 production. Furthermore, enhancement of IgE production and dermatitis by ?-toxin were abrogated in MC-deficient mice and restored by MC reconstitution. In this application, we propose three specific Aims to understand how S. aureus is sensed by skin cells including keratinocytes to induce inflammation and IgE in the skin. Furthermore, we propose studies to link S. aureus ?-toxin and related PSMs to host immune signaling pathways that mediate skin inflammation. Understanding how S. aureus ?-toxin and related PSMs regulated by the Agr quorum sensing system contribute to allergic skin disease is expected to provide critical insight into the pathogenesis of AD and the development of new therapeutic approaches to prevent and/or treat AD.